Oil is a nonrenewable natural resource having great importance to the industrialized world. The increased demand for and decreasing supplies of conventional oil has led to the development of alternative sources of crude oil such as oil sands containing bitumen or heavy oil and to a search for new techniques for more complete recovery of oil stranded in conventional oil deposits.
The Athabasca oil sands are a prime example of a huge alternative source of crude and is currently thought to have proven reserves of over 175 billion barrels recoverable by both surface mining and in-situ thermal recovery methods. There are also equally large untapped reserves of stranded light and heavy oil deposits from known reservoirs throughout North America which cannot be recovered by surface drilling methods. These two sources of oil, bitumen and stranded oil, are more than enough to eliminate dependence on other sources of oil and, in addition, require no substantial exploration.
U.S. patent application Ser. No. 11/441,929 filed May 27, 2006, entitled “Method for Underground Recovery of Hydrocarbons” discloses a method for installing, operating and servicing wells in a hydrocarbon deposit from a lined shaft and/or tunnel system that is installed above, into or under a hydrocarbon deposit. The entire process of installing the shafts and tunnels as well as drilling and operating the wells is carried out while maintaining isolation between the work space and the ground formation. This invention often involves tunnels being driven into a hydrocarbon deposit for the purpose of installing horizontal and inclined wells into the hydrocarbon deposit. These wells may be used to collect hydrocarbons and inject fluids such as gas, water, steam or diluents into the formation to apply secondary and tertiary recovery techniques. The effectiveness of collector and injector wells is directly related to the local ground permeability around the wells and to the permeability of the hydrocarbons throughout the reservoir.
Reservoir Permeability
Stranded light and heavy oil reservoirs in sandstones or carbonates may have permeabilities parallel to the bedding planes in the range of a few millidarcies to several hundred millidarcies. Permeabilities vertical to the bedding planes are in the same range but in addition there may be impermeable layers, taken here as less than about 5 millidarcies, parallel to the bedding planes that prevent continuous vertical flow of fluids.
The permeability of oil sands is typically in the range of a few hundred darcies to several darcies. However the permeability vertical to the bedding planes of an oil sands reservoir may be disrupted by impermeable beds of mudstone and shale. The permeability of these layers is typically in the range of a few millidarcies or less. These layers may be a few centimeters to several centimeters thick and can form an impermeable barrier. Alternately, these layers may not be continuous but may be comprised of many thin layers in a sequence that can form a labyrinth that has the same blocking effect as a continuous impermeable membrane.
Tunneling and Drilling
In recent decades, there has been a substantial increase in the number of soft-ground civil tunneling projects utilizing the proven technologies of tunneling and tunnel boring machines (“TBMs”). This increase is largely due to the technological development of large slurry and Earth Pressure Balance (“EPB”) tunnel boring machines. This new generation of soft ground tunneling machines can now overcome ground conditions that until now were too costly and impractical to undertake.
In particular, the civil soft-ground tunneling industry has developed methods to control ground subsidence to a few inches or less as a requirement for tunneling under buildings, utilities and other sensitive structures. This control is achieved through careful tunneling practice coupled with a variety of measurements and often supporting numerical computational analyses.
In an EPB machine, the excavated material or muck is ingested into a chamber which is maintained at about local formation pressure (hence the name earth pressure balance). The excavated material is mixed with a plasticizer that gives the muck cohesion. A screw auger then transfers the plasticized muck to a conveyor system where the muck in the auger forms an effective seal between the inside of the machine and the formation outside. A slurry TBM can also be operated to maintain the excavated material or muck at about local formation pressure in a slightly different way during excavation. The slurry TBM cutting head excavates by forming the ground just ahead of it into a dense slurry. The slurried muck is ingested into a pressurized chamber and then formed into a transportable slurry by adding additional water. The slurry may be transported out of the tunnel at approximately formation pressure in a closed slurry system. Thus, like the EPB machine, the excavation and muck removal can be carried out by a slurry TBM at or near formation pressure while the working areas in the TBM and tunnel can remain at ambient pressure and isolated from the slurried muck.
These machines are typically used to install lined tunnels and are operated to excavate the minimum size opening to efficiently install a liner. There is typically a small gap between the outside of the tunnel liner and the excavated opening. This is typically backfilled with a grout to eliminate any void space so that, among other things, the ground will not subside into the gap.
Developments in soft-ground tunneling led to the practice of micro-tunneling which is a process that uses a remotely controlled micro-tunnel boring machine typically combined with a pipe-jacking technique to install underground pipelines and small tunnels. Micro-tunneling has been used to install pipe from twelve inches to twelve feet in diameter and therefore, the definition for micro-tunneling does not necessarily include size. The definition has evolved to describe a tunneling process where the workforce does not routinely work in the tunnel.
Drilling technologies for soft and hard rock are also well known. Conventional rotary drilling and water jet drilling, for example, have been utilized in oil and gas well drilling, geothermal drilling, waste and groundwater control as well as for hard rock drilling. Drill holes are typically drilled to a diameter just large enough to allow a casing to be installed. Certain drilling techniques, such as water jet drills can be operated to over-excavate the diameter of the drill hole to a size much larger than the drill bit. Most drilling techniques, including rotary and auger, can excavate a hole somewhat larger than the casing.
In open-hole drilling with liquid drilling muds, the wellbore fluid pressure is typically maintained higher than the local natural formation pressure. This is known as overbalanced drilling and there are often problems associated with overbalanced drilling such as lost circulation, differential sticking, low drilling rates, formation damage such as development of a “skin”. Many drillers are now using underbalanced open-hole drilling where the wellbore fluid pressure is maintained lower than the local natural formation pressure, thus permitting the formation fluids to tend to flow towards the wellbore while drilling. Underbalanced drilling is considered, in many drilling situations, to reduce the problems associated with overbalanced drilling.
Although most tunneling and drilling methods are designed to form an excavated opening of the minimum diameter for installing a tunnel liner or a well casing, the technologies developed to control excavated hole diameter and ground motion can be used to create excavated openings of much greater diameter.
Means of Altering Reservoir Permeability
One of the common methods of increasing horizontal permeability is by applying well-fracturing or “fracing” methods, typically to induce fractures parallel to the bedding planes to connect pre-existing fractures and flow pathways in the reservoir rock with larger, man-made fractures. A man-made fracture starts at the well bore and extends out into the reservoir rock for as much as several hundred feet or more. In hydraulic well-fracturing practice, large volumes of fluids (typically water or water-based liquids, including polymers, in amounts from a few hundred to over 100,000 gallons) are pumped down a well bore casing. The fluid is injected into the hydrocarbon formation through perforations in the casing walls along the producing zone of the well or from a packer system inserted into open, uncased formation. The fluid pumping continues until the hydraulic pressure of the fluid in the reservoir exceeds the bulk tensile strength of the host rock, which opens up and drives fractures into the reservoir with the wings of the fracture extending away from the wellbore in opposing directions according to the orientation of natural stresses within the formation. A proppant material, such as natural or manufactured sand, is usually contained in the injected fluid to keep the fractures open after the fluid injection pressure has diminished. These induced fractures create secondary permeability by opening up pathways to connect pre-existing fractures and flow pathways in the reservoir rock and so must compact other portions of the reservoir material to conserve mass (in some cases of shallow fracturing, the well-fracturing process can produce heave of the formation at the surface). These fractures also increase the amount of area in the reservoir to which product fluids can flow, comparable to a system of multiple additional wellbores within the formation. Other means of well-fracturing include pressurizing the well-bore and any pre-existing fractures in the reservoir using propellant or explosive charges. These approaches are more dynamic and are thought to be more effective for controlling the direction of the induced fractures.
These means of increasing permeability in general do not do so by removing reservoir matrix material but act by compressing portions of the reservoir material so that the other portions may expand.
In the case of oil sands where the hydrocarbons are immobile, various thermal or chemical means may be used to melt or mobilize the bitumen or heavy oil thus enabling the natural permeability of the reservoir to be realized. Thermal treatments or water pressure treatments may also increase reservoir permeability by causing expansion of the reservoir matrix.
There thus remains a need for new methods to increase reservoir permeability that can be used in lieu of or in addition to the various reservoir fracturing methods.